1. Field of the Invention
This invention relates to a glass substrate for a magnetic disk. More specifically, this invention relates to a glass substrate of this kind which has a predetermined composition and is formed with protrusions on a surface thereof by irradiating a laser beam on the surface to form texture.
2. Prior Art
A fixed magnetic disk unit employs a mechanism called a CSS (Contact Start Stop) system in which a magnetic head is in contact with a disk surface when the disk is stationary and is brought into sliding contact with the same when the disk is started and stopped.
In the CSS system, to prevent stiction from occurring when the disk is started and stopped as well as to reduce friction which will occur on such occasions, so-called texture, i.e. a suitably finely-roughened surface formed by protruded portions and recessed portions (which may be formed by protruded portions alone) is provided on the disk. The texture is provided on all or part of a main surface of the disk. Assuming that the texture is formed merely on part (CSS zone) of the main surface, the magnetic head is moved to the CSS zone at proper timing when the disk is brought into a CSS operation. Further, when the power is turned off during rotation of the disk, the magnetic head is also moved to the CSS zone.
Particularly, when the texture is formed on part of the main surface, the remaining part can preserve smoothness similar to that of a mirror surface, which enables the magnetic head to be floated at a lower position. Therefore, the partially-formed texture is suitable for increasing the recording density (packing density) of the magnetic disk unit.
For a substrate of this disk, an aluminum (Al)-magnesium (Mg) alloy substrate plated with nickel (Ni)-phosphorus (P), i.e. a so-called aluminum substrate, has been widely employed. To provide texture on the aluminum substrate, a method of forming concentric scratches on the substrate by using an abrasive tape has been widely in practice. However, this method is difficult to meet the requirements of prevention of stiction and reduction of friction at the same time when a further lower floating position is demanded of the magnetic head.
To solve this problem, U.S. Pat. Nos. 5,062,021 and 5,108,781, for instance, disclose a process of forming pits comprised of recesses and annular protrusions surrounding the recesses on a surface of a metal of an aluminum substrate by using a Nd:YAG laser so as to reduce stiction. The above US patents merely relate to texture processing of an aluminum substrate.
On the other hand, as a method for forming texture on a glass substrate having excellent smoothness by abrasion, rigidity and impact resistance as compared with an aluminum substrate, there have been known
A. a method of subjecting a glass to dry etching by using the photolithographic method ((1) Noboru Kawai et al., Japan Lubrication Society, Draft Collection of Tribology Meeting at Fukuoka, (October 1991) p. 265, (2) H. PA1 B. a method of chemically etching a glass substrate (Japanese Laid-Open Patent Publication (Kokai) No. 3-245322), PA1 C. a method of dispersing fine particles on a glass substrate (Japanese Laid-Open Patent Publication (Kokai) No. 2-128318) and PA1 D. a method of utilizing an island-like structure by sputtering (Japanese Laid-Open Patent Publication (Kokai) No. 3-73419).
Tanaka et al., "IEEE Transactions on Magnetics" vol. 29, No. 1 (January 1993) p. 270, (3) H. Ishihara et al., "Wear" vol. 172 (1994) p. 65),
The above Method A has a characteristic that the shape or the like of texture can be controlled precisely, but the cost is high. Methods B, C and D are advantageous in the point of the cost, but they have problems that stability at the time of manufacture is slightly poor and it is difficult to form texture merely on a CSS region.
To solve the above problems, as a method for forming texture on a glass substrate, a method of using irradiation of a laser beam has recently been proposed (Japanese Laid-Open Patent Publications (Kokai) Nos. 4-311814, 7-182655 and 6-290452).
In Japanese Laid-Open Patent Publication (Kokai) No. 7-182655, it is merely described that a laser beam is employed which has a certain range of an optical transmittance of a laser beam energy pulse against the glass, but there is no description concerning the relation of the laser beam with a glass composition. Further, as to a laser beam wavelength, only a value of 10.6 .mu.m is disclosed, but no other wavelength is mentioned.
It is known that in general, in a texture part of the magnetic disk, so long as a ratio of an area of protruded portions of the texture to a total area of the surface of the magnetic disk is identical, protrusions of the texture each having a smaller diameter, i.e. protrusions of the texture with smaller space intervals are more suitable for a lubricant to take effect, thereby improving abrasion resistance of the magnetic disk ((1) Hiroshi Tani et al., Japan Tribology Society, Draft Collection of Tribology Meeting at Kanazawa, October 1994, p. 153, (2) H. Ishihara et al., "Wear", vol. 172 (1994) p. 65). Therefore, it is preferred that each protrusion of the texture has a smaller diameter than one disclosed in an example of a smaller diameter of texture is desired from the example of Japanese Laid-Open Patent Publication (Kokai) No. 7-182655 (in which each protrusion of texture has a diameter of 30 .mu.m).